Inelastic-electron-tunneling spectroscopy (IETS) is applied to investigate the spin-dependent tunneling process for a Ta/Ni80Fe20/FeMn/Ni80Fe20/Al-oxide/Co/Ni80Fe20 ferromagnetic tunnel junction. IET spectra for both parallel and antiparallel magnetization configurations of ferromagnetic electrodes exhibit a strong peak at 20 mV. Using the subtraction between these spectra, we obtain the inelastic excitation spectrum induced only by the magnetic origin. This spectrum exhibits a strong peak at 18 mV and decreases monotonically with increasing bias voltage. The bias-voltage dependence of tunnel magnetoresistance (TMR) is the same. These results are discussed by considering the distribution of the surface magnon excitation energy.